1. Field of the Invention
The present invention relates to an injection mold for injection molding substrates for high-density recording media including ROM disks, such as CD's and VD's, and E-DRAW disks.
2. Background Art
Although glass, metals and ceramic materials are used for forming substrates for optical disks, plastic materials are preferable in respect of weight, strength and productivity. However, since the plastic substrate for an optical disk is very thin and flat, having a thickness in the range of 1.2 to 1.5 mm and a diameter of approximately 300 mm at the maximum, and when a plastic material, such as an acrylic resin or a polycarbonate resin, having low fluidity is used, it has been difficult to form a satisfactory plastic substrate by injection molding.
Among the requisite properties of the substrate for optical disks, the capability of reproducing the surface shape of a stamper placed within the mold is the most important property. Generally, the condition, or quality, of reproduction of the surface shape of the stamper on the substrate is dependent on the injection pressure. That is, the higher the injection pressure, the better the transfer of the submicroscopic surface pattern of the stamper for a high-density information recording medium to the substrate. Actually, to improve the transfer of the minute surface shapes in the peripheral portion of the stamper in a thin and flat cavity, high injection pressure is necessary. However, since the disk has a small weight, generally, a disk molding machine capable of applying only a small clamping pressure is employed for molding the disk in spite of the cavity having a large area extending perpendicularly to the direction of the clamping pressure. Accordingly, when a high injection pressure is applied to the injection mold to achieve the satisfactory transfer of the minute surface shapes of the stamper to the disk, the sections of the injection mold are separated from each other along the parting line, and thereby burrs or flashes are formed in the molding.
Generally, the conventional injection mold for injection-molding a plastic substrate for an optical disk has a construction as illustrated in FIGS. 1 or 2.
Referring to FIG. 1, a mold has a molding cavity 3 defined by the inner surfaces of a pair of half molds, for example, a movable half mold 1 and a fixed half mold 2. A stamper 4 having submicroscopic information pits or information tracks is secured to part of the surface of the molding cavity, i.e., the movable mold 1, with a stamper holding ring 5. A ring 6 (for holding another stamper, not shown) is attached opposite the stamper holding ring 5 to the surface of the molding cavity of the fixed half mold. When a molten resin is injected through a sprue 7 into the molding cavity 3, the air in the molding cavity is discharged outside through the gap between the respective surfaces of the rings 5 and 6, and the gap along the parting line 8.
Since the mold having a construction as shown in FIG. 1 opens, i.e., the sections of the mold separate, when a high injection pressure is applied thereto, the molten resin flows into the gap between the rings 5 and 6 to form burrs or flashes. The burrs and flashes require disadvantageously additional burring work and makes the molding cavity dirty, which is a significant problem in manufacturing optical disks which require the utmost cleanliness of the molding cavity.
Another conventional injection mold is shown in FIG. 2, in which members corresponding to those shown in FIG. 1 are designated by the same reference numerals with a prime. The injection mold of FIG. 2 is not provided with the ring 6. In the injection mold of FIG. 2, the circumference 9 of a projection formed in the fixed half mold 2' abuts with the tapered inner circumference of a stamper holding ring 5' to define the molding cavity. This construction does not have the problem of burrs as does the construction shown in FIG. 1. However, since the tapered inner circumference 10 of the stamper holding ring 5' and the circumference 9 of the projection of the fixed half mold 2' must be finished with a very high machining accuracy for a precision fit, it is difficult to fabricate the injection mold and there is a high possibility of seizure occurring between the circumference 9 and the tapered inner circumference 10. Furthermore, since a vent for discharging gas needs to be formed between the circumference 9 and the tapered inner circumference 10, a clearance which will not allow burrs to be formed must be formed between the circumference 9 and the tapered inner circumference 10. Accordingly, in designing the injection mold, two conditions contradictory to each other, namely, forming a clearance as a vent and the preventing of burrs, must be satisfied. This greatly restricts the freedom of design of the mold. Still further, in adjusting the height of the stamper holding ring to regulate the clearance for forming an appropriate vent, the stamper is frequently compressed an excessive amount, and thereby the expansion and contraction of the stamper are restricted. Consequently, the life of the stamper is greatly reduced.
Accordingly, it is an object of the present invention to provide a novel mold which does not have the disadvantages of the known molds, is capable of preventing burrs or flashes, has an enhanced degree of freedom of design, does not have a possibility of seizure, and allows the free expansion and contraction of the stamper.